FORMATION OF THE EMBRYO. 737 



tinction between their central portion or nucleus, and their external 

 portion or shell. The nucleus is more transparent, and will afterward 

 supply the cartilaginous deposit for the permanent vertebrae ; the shell 

 has a radiating striated texture, and serves for the formation of mus- 

 cular tissue. 



On the second day of incubation (Fig. 252, I.) the primitive verte- 

 brae, as seen in transverse section, have the form of a narrow oval, with 

 a small nucleus and a comparatively thick and perfectly continuous 

 shell. From the second to the third day (Fig. 252, II.) the nucleus 

 grows more rapidly than the outer parts, which it pushes upward and 

 downward ; and the shell begins to show indications of a separation 

 into upper and lower portions. On the third day (Fig. 252, III.) this 

 separation is complete ; and the upper portion of the shell, taking a 

 position more or less parallel with the outline of the body at this point, 

 will become the layer of voluntary muscles about the spinal column. 

 Its lower portion recedes farther from above downward, and approaches 

 the situation of the double aorta ( 8 ), which it will afterward supply with 

 its involuntary muscular layer. In a section of the embryo at the 

 fourth day (Fig, 252, IV.) the final position of these two muscular 

 layers is distinctly marked ; the projection of the spinal ridge, on the 

 one hand, having become higher and steeper, and, on the other, the 

 double aorta having been fused into a single vascular canal. 



The nucleus of the primitive vertebra, in the mean time, extends 

 upward and inward, in such a manner as to surround both the medul- 

 lary canal and the chorda dorsalis, which it embraces in a tissue of new 

 formation. This tissue afterward supplies the cartilage, both of the 

 bodies of the vertebrae, and of the oblique processes which inclose the 

 spinal canal at its sides and behind. But when these cartilages are 

 formed, it is observed that they do not correspond in situation with the 

 original primitive vertebrae. A new segmentation takes place, by which 

 the lines of separation between the successive permanent vertebrae pass 

 through the middle of what were the primitive vertebrae j 1 and conse- 

 quently each permanent vertebra is formed out of the adjacent halves 

 of two primitive vertebrae. The chorda dorsalis, included in the car- 

 tilaginous matrix of the bodies of the vertebrae, ceases to grow in a 

 corresponding ratio with the neighboring parts, becomes atrophied, and 

 disappears ; while the bodies of the vertebrae, which surround it, are 

 rapidly enlarged, and assume the form and size of the principal com- 

 ponent parts of the spinal column. 



1 Foster and Balfour, Elements of Embryology. London, 1874, p. 153. 



